1. Technical Field
The invention relates to semiconductor processing equipment. More particularly, the invention relates to an encapsulated screw, especially for use in a semiconductor wafer processing chamber in connection with a wafer clamp ring.
2. Description of the Prior Art
FIG. 1 is a cross sectional side elevation view of a wafer chuck 10 and clamping assembly 12. The chuck shown for purposes of example in the figure includes a flat upper surface 19 that is adapted to support a semiconductor wafer 16 during wafer processing. Such processing may involve such known techniques as ion implantation, CVD, PVD, or reactive ion etching. Although the examples provided herein are directed to processes involving annealing and reflow and not deposition, it will be appreciated by those skilled in the art that such wafer processing equipment may be used for many other processes.
The clamping assembly 12 secures the wafer 16 to the wafer chuck surface 19 during wafer processing. Typical wafer clamping assemblies are formed as ring-shaped members having an open inner portion 25 that allows access to the wafer surface during the various processing steps. The clamping assembly also includes a locating flange 27 that is configured to rest in a complementary slot 29, 31 during wafer clamping. A lift assembly, including a lift bracket 33 and lift finger(s) 17, is provided to raise and lower the wafer relative to the wafer chuck surface 19 and thereby allow the wafer 16 to be placed on and removed from the wafer chuck 10.
An important function of the wafer chuck is to provide thermal mass that precisely maintains the wafer at a desired temperature. Heating elements 11 are provided to raise the wafer temperature. For example, the heating elements may be provided as a dual-zone heater, i.e. a heater having two heating zones, or as a five-zone heater, i.e. a heater having five heating zones.
A thermal transfer gas may be supplied to a backside of the wafer through a gas conduit 15 to heat the wafer, and a cooling fluid, such as water, can be supplied to the wafer chuck through a fluid conduit 21. A thermocouple 13 that monitors the temperature of the wafer chuck surface 19 is used to control the temperature of the wafer by controlling the amount of heat and/or cooling supplied to the wafer by the heating elements 11, thermal transfer gas conduit 15, and/or cooling fluid conduit 21. The heating elements may be arranged in any of several known configurations.
In operation the clamping assembly 12 is first raised from the wafer chuck 10 by a lifter (not shown). The wafer 16 to be processed is then slid into position above the wafer chuck by a robot arm (not shown) and placed on several lift fingers 17. The lift fingers are retracted, thereby lowering the wafer onto the wafer chuck surface 19. The clamp ring is then lowered into position and wafer processing may begin.
The clamp ring must be mounted to the assembly. This is accomplished by the use of a screw or other such fastening mechanism, although a screw and lock washer is presently the most common method of accomplishing such mounting. In a typical application, up to twelve or more screws may be used to hold the clamp ring to the assembly.
While a screw is a simple and effective expedient for securing the clamp ring to the assembly, such screw is also a source of contamination within the processing environment. This is due, in part, to the fact that the screw is typically formed of a material that is not process compatible, i.e. a metal. To avoid poisoning the process, the screw is usually provided as part of a compound assembly that includes the screw, a lock washer, a flat washer, and a process compatible cap.
While variations on this arrangement may be found in the industry, it is almost always necessary to shield the screw from the process environment. In doing so, the screw mechanism becomes unnecessarily complex. Not only is the screw provided with a complement of washers, but the process compatible cap, while somewhat preventing exposure of the screw itself to the process environment, nonetheless becomes a significant source of contamination.
In particular, the cap must be seated on the head of the screw. Thus, the screw is first driven into the clamp ring to effect securement thereof, and the cap is then compression fitted to the screw. To do so, the four components of the compound screw assembly must be handled, and it is not unusual that one or more of the screw elements is dropped or lost during such procedure.
More significantly, because this arrangement does not provide an intimate airtight seal of the cap to the screw head, particulates can accumulate at the interface of the cap and the clamp ring. These particulates are a source of contamination. Further, the cap can become dislodged during processing, thereby exposing the screw to the process environment.
It would be advantageous to provide an improved, process compatible fastener that would allow a clamp ring to be secured to an assembly for use in a process environment. It would be further advantageous if such fastener were of a simple design, such that it is readily manufactured, and such that it is not a source of contamination.